Cucurbit Downy Mildew Update: July 9, 2025

On July 1, cucurbit downy mildew was confirmed on cucumber in Lancaster County, PA. A day later, Dutchess County in eastern New York reported downy mildew on cucumber (see map below). Depending on the prevailing weather pattern, the pathogen can move from sources up the east coast or from the west, as there were 4 counties in Michigan with downy mildew infections reported. The majority of reports have been on cucumber although there have been reports on acorn squash and watermelon in South Carolina, and pumpkin in Alabama. This means that there are sources of the pathogen that can infect all cucurbit crops coming from the west and southeast, depending on the forecasted weather conditions. It is highly recommended that all cucumber and cantaloupe crops be protected with fungicides, even those being grown in high tunnels. The high relative humidity in a high tunnel can be enough for downy mildew to develop. In order to be used in a high tunnel, the product must be labeled for greenhouse use. See Table E-12 in the Pest Management section of the 2045-2025 Mid-Atlantic Vegetable Production Recommendations for a listing of products labeled for greenhouse production.

Downy mildew is caused by the oomycete pathogen, Pseudomonas cubensis. Over the past several years differential disease susceptibility between cucurbit hosts, as well as differential efficacy of targeted fungicides depending on the cucurbit host, have been observed and has led to dividing the pathogen into two clades. Pathogen isolates in one of the clades primarily affect cucumber and cantaloupe while isolates from the other clade infect pumpkin, squash, and watermelon. 

Preventive applications are much more effective than applications made after the disease is detected. Since fungicides are the primary management tool for downy mildew, it is important to have thought through a fungicide program that rotates among different FRAC codes for fungicide resistance management as well as tank mixes with protectant fungicides that contain chlorothalonil (FRAC M05) and mancozeb (FRAC M03) if not already in the premix. Downy mildew targeted fungicides include but are not limited to, Orondis Opti (FRAC 49 + M05), Ranman (FRAC 21), Elumin (FRAC 22), Zampro (FRAC 40 + 45), etc. See the 2024-2025 Mid-Atlantic Commercial Vegetable Production Recommendations for a more complete list of fungicide recommendations. Remember that fungicides that specifically target downy mildew will not manage powdery mildew.

Other foliar diseases that could be confused with downy mildew when scouting include angular leaf spot, anthracnose, Alternaria leaf blight, and powdery mildew.

Extension educators are visiting farms and scouting for disease. Please email Leah Fronk at lxf339@psu.edu to contribute to this report. The next report will be posted on July 17, 2025.